tests: add test suite for jay_diff; fix two bugs found

Move the inline assertions from jay_diff.py into tests/test_jay_diff.py
as proper pytest functions, grouped by function under test.  Add corner
cases for numeric/bool/None/list values, completely different structures,
empty containers, deep nesting, type changes (dict↔scalar), and
parametrised round-trip tests (diff→merge produces new).

Two bugs fixed during the exercise:
- jay_diff_rev: guard against _old being a non-dict (type change from
  dict to scalar caused TypeError on item assignment)
- jay_merge_delete / jay_merge_update: already fixed in previous commit;
  covered by new tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/jay_diff.py

@@ -3,6 +3,8 @@ from copy import deepcopy
 
 def jay_diff_rev(_old, _new):
 	delta = {}
+	if not isinstance(_old, dict):
+		return delta
 	for k in _new.keys():
 		if k in _old:
 			if isinstance(_new[k], dict):
@@ -131,7 +133,7 @@ def jay_merge_full(_old, _diff):
 
 	return res
 
-def	main() -> None:
+def _legacy_assertions() -> None:
 	result = jay_diff_add({}, {'a': 'hamburger', 'b': 'fries', 'c': {'e': 'coke', 'f': 'coffee'}})
 	assert result == {'a':'hamburger', 'b':'fries', 'c': {'e': 'coke', 'f': 'coffee'}}
 
@@ -579,4 +581,4 @@ def	main() -> None:
 	assert result == old
 
 if __name__ == '__main__':
-	main()
+	_legacy_assertions()